1. Field of the Invention
The present invention relates to an evoked response detector for a heart stimulator, the heart stimulator having a pulse generator for producing stimulation pulses for delivery to the heart of a patient through a lead intended to be connected to the pulse generator and introduced into the heart, the evoked response detector having measuring means for measuring electrode signals picked up by the lead.
2. Description of the Prior Art
To reduce the energy consumption of heart stimulators, an automatic threshold search function, a so called AUTOCAPTURE™ function, is used to maintain the energy of the stimulation pulses at a level just above that which is needed to effectuate capture; cf. e.g. U.S. Pat. No. 5,458,623. A reliable detection of the evoked response, which then is necessary, is, however, not a simple matter, especially when it is desired to sense the evoked response with the same electrode as the one delivering the stimulation pulse and in particular if the sensing is performed by a unipolar electrode configuration.
Currently, fusion beats create a significant problem for the AUTOCAPTURE™ function since these beats often are not detected as heart beats. Instead the heart stimulator in question interprets the evoked response as a loss of capture and as a consequence a backup pulse is issued and the stimulation pulse amplitude is increased. After several such undetected fusion beats the heart stimulator will be in a high output mode, where it remains until the next threshold search is performed. This misinterpretation by the heart stimulator of the evoked response signal will, of course, increase the current drain and decrease the lifetime of the battery and the AUTOCAPTURE™ function will be disabled for some time.
In European Application 0 836 867 a heart stimulating device for avoiding fusion beats is disclosed. Thus, a technique is described for improved detection of intrinsic events. A control means is then activating a non-filtered sensing means before the end of a basic escape interval and is prolonging this escape interval by a predetermined length if the non-filtered sensing means senses a QRS complex, such that the corresponding filtered signal, which is delayed due to the filtering procedure, has time to reach the control means for safe verification of the sensed QRS complex.
In U.S. Pat. No. 4,757,815 is disclosed a heart pacemaker including a pulse generator and a circuitry for measuring a respiration signal of a patient and a control unit for controlling the pulse generator by changing the pulse repetition rate dependent on the respiration signal. In this known device a series of heart action signals are acquired by e.g. a QRS-detector. The amplitudes (R-waves) of the heart action signals are subject to fluctuations caused by the respiration cycle of the user of the heart pacemaker. The respiration measurement is undertaken by measuring, for each QRS-complex, the distance between the most positive point and the most negative point of the complex, whereas the distance variation between consecutive measurements being a measure of the respiration of the patientAccording to this known device two separate points of the QRS-complex must be identified and determined, this is achieved by a peak sensing means including sample and hold circuits.
In U.S. Pat. No. 5,391,192 is disclosed a clinical programming system for use with an implanted cardiac system to automatically determine the minimum energy required to evoke a ventricular depolarization. Evoked response is detected through evaluation of the integral of R-waves provided by a surface electrocardiogram. U.S. Pat. No. 5,391,192 makes no attempt to differentiate between fusion beats and true captured beats.